Lower Respiratory Tract: Respiratory Zone

1. Respiratory Zone

In contrast to the conducting zone, the respiratory zone includes structures that are directly involved in gas exchange. The respiratory zone begins where the terminal bronchioles join a respiratory bronchiole, the smallest type of bronchiole, which then leads to an alveolar duct, opening into a cluster of alveoli.

1a. Alveoli

An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli. An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts and it performs gas exchange in the lungs.

An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange. An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange. Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung.

The alveolar wall consists of three major cell types: type I alveolar cells, type II alveolar cells, and alveolar macrophages. A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitutes up to 97% of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases. A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant, a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli. Roaming around the alveolar wall is the alveolar macrophage, a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli.

The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries. Taken together, the alveoli and capillary membranes form a respiratory membrane that is approximately 0.5 μm (micrometers) thick. The respiratory membrane allows gases to cross by simple diffusion, allowing oxygen to be picked up by the blood for transport and CO₂ to be released into the air of the alveoli.

EXAMPLE

Respiratory Disease: Asthma

Asthma is a common condition that affects the lungs in both adults and children. In 2021, approximately 8% of adults (20.3 million) and 6.5% of children (4.6 million) in the United States suffered from asthma (CDC, 2023). In addition, asthma is the most frequent cause of hospitalization in children.

Asthma is a chronic disease characterized by inflammation and edema of the airway, and bronchospasms (that is, constriction of the bronchioles, or bronchoconstriction), which can inhibit air from entering the lungs. In addition, excessive mucus secretion can occur, which further contributes to airway occlusion. Cells of the immune system, such as eosinophils and mononuclear cells, may also be involved in infiltrating the walls of the bronchi and bronchioles.

Bronchospasms occur periodically and lead to an “asthma attack.” An attack may be triggered by environmental factors such as dust, pollen, pet hair, or dander, changes in the weather, mold, tobacco smoke, and respiratory infections, or by exercise and stress.

Symptoms of an asthma attack involve coughing, shortness of breath, wheezing, and tightness of the chest. Symptoms of a severe asthma attack that require immediate medical attention would include difficulty breathing that results in blue (cyanotic) lips or face, confusion, drowsiness, a rapid pulse, sweating, and severe anxiety.

However, the severity of the condition, frequency of attacks, and identified triggers influence the type of medication that an individual may require. Longer-term treatments are used for those with more severe asthma. Short-term, fast-acting drugs that are used to treat an asthma attack are typically administered via an inhaler to promote bronchodilation (widening of the airways). For young children or individuals who have difficulty using an inhaler, asthma medications can be administered via a nebulizer.

In many cases, the underlying cause of the condition is unknown. However, recent research has demonstrated that certain viruses, such as human rhinovirus C (HRVC), and the bacteria Mycoplasma pneumoniae and Chlamydia pneumoniae that are contracted in infancy or early childhood, may contribute to the development of many cases of asthma.

terms to know

Respiratory Bronchiole

A specific type of bronchiole that leads to alveolar sacs.

Alveolus

A small, grape-like sac that performs gas exchange in the lungs.

Alveolar Sac

A cluster of alveoli.

Alveolar Pores

An opening that allows airflow between neighboring alveoli.

Type I Alveolar Cell

Squamous epithelial cells that are the major cell type in the alveolar wall; highly permeable to gases.

Type II Alveolar Cell

Cuboidal epithelial cells that are the minor cell type in the alveolar wall; secrete pulmonary surfactant.

Pulmonary Surfactant

The substance composed of phospholipids and proteins that reduces the surface tension of the alveoli; made by type II alveolar cells.

Alveolar Macrophage

The immune system cell of the alveolus that removes debris and pathogens.

Respiratory Membrane

The alveolar and capillary wall together, which form an air-blood barrier that facilitates the simple diffusion of gases.

Asthma

A chronic disease characterized by inflammation and edema of the airway as well as bronchospasms.

Bronchoconstriction

Decrease in the size of bronchioles caused by constriction of the muscular walls.

Bronchodilation

Increase in the size of bronchioles caused by relaxation of the muscular walls.